We previously showed that the chondrocyte proliferative response to TGFbeta decreases with aging while, in contrast, levels of TGFbeta induced pyrophosphate (PPi) increase. Chondrocyte apoptosis has been identified as a regular feature in OA. Chondrocyte death not only results in a loss of the cells that are responsible for maintenance of cartilage matrix but also contributes to aging-associated modifications of extracellular matrix. Studies on the regulation on chondrocyte apoptosis identified death receptor activation and mechanical stress as inducers of apoptosis. Cytokine-induced anti-apoptotic mechanisms and integrin- mediated interactions with matrix promote chondrocyte survival. Our hypothesis proposes that as a function of aging, cartilage cellularity decreases, because of the reduced cell replication and increased chondrocyte apoptosis. Cell death-related and independent changes impair biomechanical properties and the survival promoting function of cartilage matrix. Intracellular aging-associated changes, specifically affecting cell cycle regulator not only account for the decrease in cell proliferation but also affect gene expression and cell survival. To test this hypothesis the specific aims will: 1. Analyze cartilage cellularity, chondrocyte apoptosis and the presence of apoptotic bodies in human cartilage aging and determine correlations with degradation and calcification of cartilage extracellular matrix. 2. Investigate mechanisms of apoptosis with regard to aging associated changes in the susceptibility of chondrocyte to undergo apoptosis. Distinguish the impact of intracellular mechanisms from aging- associated change in extracellular matrix. 3. Study the mechanisms of apoptosis by mechanical stress using in vitro cartilage organ culture systems: the expression of death receptors and their ligands, and the intracellular regulators Bcl-2 and Bax. 4. Analyze the role of mitochondria in chondrocyte apoptosis and establish the profile of caspases that are activated after mechanical cartilage injury. 5. Analyze TGFbeta activated signaling events that regulate cell cycle progression, gene expression and cell survival in chondrocyte aging.